Digester



Aug. 15, 1933. J. 1.. MASON 1,922,547

DIGESTER Filed April 29. 1931 4 Sheets-Sheet l (Q g & gnvewfo'c john Z. Mason 111$ QUOTA/M130 Aug. 15, 1933. MASON 1,922,547

DIGESTER Filed April 29. 1931 4 Sheets-Sheet 2 fnizn Z. 1 .10.5011 55 1113 GHQ 044mg:

avwamfoz Patented Aug. 15, 1933 PATENT OFFlCE DIGESTER John L. Mason, York, Pa., assignor to Hardinge Company, Incorporated, York, Pa., a Corporaflon of New York Application April 29, 1931. Serial No. 533,863

4 Claims.

This invention relates to digesters for disposing of sewage and other forms of sludge. Tanks are commonly used for confining the sewage during the period of time necessary for bacterial action to take place and it has been found that the digesting action may be hastened by stirring the sludge. For this-reason and for sanitary purposes it has become the practice to provide a cover or roof for the tank in which the sludge is contained. Digester tanks are usually large and roofs or covers therefore must be especially supported to carry the heavy load incident to a broad span. While theproblem of supporting a large roof requires careful attention the problem is much more difficult of solution when it is desired to provide stirring mechanism for the contents of the tank. Large roofs and actuating mechanism for stirring apparatus may be supported by super-structure such as a trussed construction carried by the walls of the tank or by supporting pillars, but it is desirable to build digesters less expensively. Concrete is a satisfactory substance with which to construct tanks to be used for digester purposes and but little difficulty is experienced in building walls of a tank of concrete and such reinforcement as is necessary. A roof or cover made of concrete and joining the wall of the tank is capable of being made gas tight, but construction difficulties present themselves unless precaution is taken for properly supporting the concrete structure so as to prevent the appearance of cracks resulting .from tensile stresses.

Among the objects of the present invention is to provide a digester made of reinforced concrete and having a self supporting roof capable of supporting the actuating mechanism for the stirring apparatus and also the stirring apparatus. This tank and cover or roof eliminates the necessity of a center supporting column or the use of a bridge truss and is designed to withstand the varying stresses incident to roof loads and the many stresses caused by supporting actuating mechanism and a moving stirring apparatus.

Other objects involving novel construction and combinations of parts will appear in the specification and in the claims thereof, setting forth a preferred embodiment of my invention which may be fully understood from the detailed description and drawings.

In the drawings:

Fig. 1 is a vertical section taken through the center of the tank.

Fig. 2 is a plan of a portion of the roof showing part of the concrete construction broken away.

Fig. 3 is a plan of the actuating mechanism for the stirring apparatus.

Fig. 4 is a section taken on line 4-4 ofFig. 3 and showing in detail the mechanism whereby the stirring apparatus may be raised or lowered.

Fig. 5 is an enlarged view of the elevating mechanism on line 5-5 of Fig. 4.

Fig. 6 is a detail of the concrete construction at the joint of the roof and wall and on line 6-6 of Fig.2.

The general construction of the digest-er may be readily understood from Fig. l in which 10 is a tank having a wall 11, a bottom 12 and a roof 13. In view of the fact that the stirring apparatus rotates about a vertical axis the tank is preferably circular in plan. Sewage or any material which is tobe digested enters the tank through a conduit 14 and the solid matter finds its way to the bottom of the tank. In the process of digesting, the character of the solid matter is changed and scum rises to the surface of the liquid. The solid matter settles to the bottom and is drawn out of the tank through a conduit 15 which leads from a sump 16.

The tank is filled up to the roof with liquid and matter undergoing digestion and the scum which rises to the surface and into contact with the roof is Withdrawn by a conduit 17 having connection with a gas chamber 18. Gases which are formed within the tank may be taken therefrom through a pipe having connection with valve 19 which is mounted upon the gas chamber 18. Conduit 20 serves as an outlet for the liquid in the tank and this enters the tank at a point considerably below the high water line or roof of the tank. Liquid leaving the tank through conduit 20 passes over a weir 21 and is removed from a trough 22 by means of a conduit 23. 24 is a manhole, normally sealed. The entire interior of the tank is sealed from the atmosphere except for conduit 20 and this operates as a liquid seal. The concrete construction used is gas tight and water tight.

In order to hasten the bacterial action throughout the tank and in the sludge at the bottom of the tank it is of advantage to stir the same and for this purpose stirring apparatus is provided. This stirring apparatus comprises a shaft 26 which passes through the roof at 27. This shaft carries at its lower end a plurality of blades 28 which extend radially fromthe shaft 26. Any convenient form of structure may be utilized for supporting the blades 28 and this structure may take the form of bracing 29, 30, 21, made of angle irons or bars of any suitable form. A similar set of blades 33 is mounted upon the shaft 26 near the roof 13 of the tank and these blades 33 serve the purpose of preventing the caking of scum which might otherwise lodge against the roof of the tank. It is apparent that the stirring apparatus may take a varietyof forms and that the number of blades may vary in number from one to many. In the embodiment of the'invention illustrated, the stirring apparatus comprising blades 28 and blades 33 is supported by shaft 26 upon a thrust bearing 34, Fig. 4. The lower end of the shaft 26 is supported against lateral displacement by a bearing 35 which is supported 'by a spider 36 in the sump 16. The lower end of the shaft 26 carries a blade 37 by which the material in the sump is maintained in motion, Fig. 1.

Referring particularly to Fig. 3, the thrust bearing 34 is formed integrally with a cast iron block 39 which rests upon beams 40, 41. Beams 40, 41 are carried by stringers 42, 43 resting upon a concrete support 44 extending from the roof of the tank. The entire weight of the stirring apparatus is carried by shaft 26 and thrusts caused by this weight are carried by the roof of the tank through the thrust bearing 34 and supporting structure and support 44. The basin 45, when containing water, serves as a seal for shaft 26.

Shaft 26 is supported upon thrust bearing 34 by means of a threaded bushing 46 engaging a threaded portion 47 of the shaft. This threaded bushing 46 rests upon the hub 48 of a worm wheel 49 which, in turn, bears directly upon the upper race 50 of the thrust bearing 34. The threaded bushing 46 and threads 47 afford a means by which the elevation of the shaft 26 and stirring apparatus may be changed. The elevating mechanism is clearly illustrated in Figs. 3, 4 and 5. A toothed wheel 51, which is keyed to the threaded bushing 46, may be rotated in eitherdirection through the medium of a pivoted dog 52 and'by means of a handle 53. Depending upon the direction of rotation desired of the toothed wheel 51 and threaded bushing 46, the pivoted dog 52 is caused to engage the teeth on the threaded Wheel 51 by tooth 54 or by tooth 55. The rotation of the handle 53 in one direction causes the shaft 26 to be raised and in the other direction permits the shaft 26 to descend. This elevating mechanism is of particular use when the tank is first constructed. The floor 57 of the tank may be made from grouting and its surface is leveled byrotating the blades 28 just above the surface of the concrete bottom 12. After the floor of the tank has set and the tank is ready for use the stirring apparatus may be raised to such an extent as to permit the blades 24 to move over the surface of the floor without touching it.

The actuating mechanism for the stirring apparatus consists of a motor 60 and gearing 61, including a sprocket chain 62 and a worm 63 engaging worm wheel 49. Worm wheel 49 is keyed to shaft 26 and as the motor is operated the shaft 26 will be rotated at a slow speed. Mechanisms of this type do not require that the speed of the stirring apparatus be more than sufficient to prevent the caking of solid material or scum.

Theworm 63 is a part of or is connected to shaft 64 which is mounted in bearing 65, 66. One end of shaft 64 carries a slip clutch 67 which serves to prevent injury to any of the parts because of the obstruction of movement of the stirring apparatus and shaft 26.

As has been explained, the cover 13 of the tank serves as the sole support for the stirring apparatus and the actuating mechanism therefor. Since concrete is weak in tension provision must be made for properly reinforcing the concrete of the roof and wall in such a manner as to counteract any tendency of the concrete being called upon to resist tensile forces. As may be seen in Figs. 1 and 6, the wall 11 of the tank is suitably reinforced by a series of steel rods 70 arranged in such fashion as to give support throughout the wall. In the embodiment of the invention illustrated, two concentric series of vertically disposed rods are used, the rods of the inner series of vertically disposed rods being joined by horizontally extending rods 71 and the rods of the outer series of vertically disposed rods '72 being likewise joined, Fig. 2. This arrangement of wall reinforcing bars may, of course, be varied to suit the design and size of the tank. Around the outside surface of the wall 11 there is a series of bars or bands 73 encircling the tank and spaced from each other from the bottom of the tank to the roof. These bands are designed for giving sufficient support to the wall to withstand the hydrostatic pressure of the liquid contained within the tank and to serve to carry all tensile stresses imposed.upon the wall because of the contents of the tank. The spacing of these rods or bands is dependent upon the amount of tensile stress which each rod is designed to withstand.

Preferably each rod or band '73 should encircle the tank and be placed in position after the concrete in the major portion of the wall 11 has had an opportunity to set. If this procedure is followed, the shrinkage of the concrete in setting will not cause an initial tension to be placed upon the concrete. After the concrete has set the rods may be tightened to a predetermined initial tensile stress by means of a turn-buckle 74 such as is illustrated in Fig. 2. The initial stress applied to each rod should equal the approximate working tensile stress in the wall of the tank in the location of each particular bar. With the wall constructed in this manner the concrete of the wall will always be under compression whether or not the tank is filled with liquid and all of the tensile stress which is ordinarily taken up in the wall of the tank will be carried by the encircling bands '72. This mode of constructing the wall of the tank is utilized in the placing of the bands '75 of the outside encircling bands at the upper portion of the wall.

In view of the fact that the upper portion of the wall and the edge of the cover of the tank is submitted to extreme tensile forces it is desirable that the. encircling bands near the roof be placed close enough together as to provide sufficient strength of the bars to counteract the extremely high tensile stresses occurring. These tensile stresses are of a varied nature because of the great weight of the roof and the apparatus which it carries and the movability of the stirring ap- Y each band. A suitable pocket in the concrete is left for enabling the manipulation of the turnbuckles. After the desired initial tension has been placed in each band the bands may be covered with concrete or left exposed except for paint or other protective covering. Because of the varied character of the stresses occurring at the edge of the roofprecaution must be taken to apply sufficient initial tensile stress to each band to insure that the concrete joining the cover in the wall may never pass into tension.

As may be readily seen in Fig. 2 the roof is provided with concentric circles of reinforced bars 76 which are joined in pairs by successive bars 77. This construction gives sumcient rigidity to the roof to maintain its arched form. So as to assist in the support of the roof at the wall, some of the bars, '78, are curved into the wall. Stirrups 79 are also provided athintervals about the periphery of the wall and these extend from the wall into the roof.

It will be seen that by the construction described, the roof and the wall are joined as an integral unit and the encircling bands '75 are so positioned and tensioned as to prevent the concrete from carrying tensile stresses.

In the foregoing I have described what I consider the preferred embodiment of my invention because the type of construction involved is well suited for large tanks as well as smaller ones. One of my primary purposes is to provide a digester tank having a cover or roof which is self-supported, i. e. which is supported at or near its edge as by the tank wall, and is capable of supporting the weight of stirring mechanism as well as all of its own weight. For small tanks, in which the span across is not excessive, some other construction may be used in both the wall and the cover. The cover may be arched or in the form of a cone and, as it is desired that it be made of concrete, it may be suitably strengthened by steel. I do not wish to be limited to the shape of the roof, for it is readily appreciated that the shape and design is a matter which may be dependable upon the costs of construction and the stresses to be counteracted.

I claim:

1. A digester comprising a circular tank having a substantially fiat bottom, a vertical side wall, and an arched roof, means for feeding liquid into the tank through the roof thereof, means for withdrawing clarified liquid from the tank through an opening in the roof thereof, said tank being adapted to be completely filled with liquid to its roof, means for sealing the interior of the tank from the atmosphere, a sump in the center of the bottom of the tank for collecting solid matter separated from the liquid, means for withdrawing the solid matter from the sump, stirring mechanism in the tank for preventing the accumulation of solid matter on the bottom and also on the underside of the roof of the tank,

said stirring mechanism comprising a vertical shaft depending from the roof of the tank and supported therefrom, a plurality of radially disposed blades mounted on the lower portion of said shaft for scraping sludge on the bottom of the tank towards said sump, and a plurality of radially disposed blades mounted on said shaft for scraping the scum from the underside of the roof, and means operatively associated with said shaft and located above said roof and supported solely thereby for rotating said shaft.

2. A digester comprising a tank having a roof for closing the top of the tank, a sump in the bottom of the tank for collecting solid matter, means for feeding liquid into the tank, means for Withdrawing liquid from the tank, saidtank being adapted to be completely filled with liquid to its roof, means for sealing the interior of the tank from the atmosphere, a plurality of blades for scraping sludge on the bottom of the tank towards said sump, a plurality of blades for scraping the scum from the underside of the roof of the tank, a shaft depending from the roof for supporting all of said blades, and means mounted above said roof and supported solely thereby for rotating said shaft and the blades carried thereby.

3. A digester comprising a tank having a bottom, side wall, and an arched roof, said tank being adapted to be completely filled with liquid to its roof, means for sealing the interior of the tank from the atmosphere, means for feeding liquid into the tank, means for withdrawing liquid from the tank, a sump in the bottom of the tank for receiving solid matter separated from the liquid, a vertical shaft depending from the roof of the tank and supported only therefrom, blades mounted on the shaft for scraping sludge on the bottom of the tank towards said sump, a second set of blades mounted on said shaft and conforming substantially to the arched shape of the roof for scraping scum from the underside of the roof. and means located above said roof and supported solely thereby for rotating said shaft.

4'. A digester comprising a tank having a bottom, side walls, and an arched roof formed integrally from reenforced concrete, means for feeding liquid into the tank through the roof, means for withdrawing liquid from the tank through an opening in the roof, said tank being adapted to be completely filled with liquid to its roof, means for sealing the interior of the tank from the atmosphere, a sump in the bottom of the tank for receiving solid matter, a vertical shaft journalled in the roof of the tank and supported solely thereby, blades fastened to said shaft for scraping sludge on the bottom of the tank towards said sump, blades fastened to said shaft and conforming to the contour of the roof for scraping scum from the underside of the roof. and means supported solely by the roof for rotating said shaft.

JOHN L. MASON. 

